The present invention relates to a drive system and a method for operating such a drive system, in particular for an electric vehicle.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
An electric vehicle is driven by way of a battery carried by said vehicle, a drive regulator, an inverter and an electric motor—hereafter referred to simply as a motor. The drive system here contains at least the drive regulator and the inverter. If a battery and motor are connected, these are also part of the drive system. The motor here is in particular a permanently excited synchronous motor. Such motors have a high level of efficiency and a high power density. A specific speed to torque characteristic means that such a motor is also operated in the field weakening range. To this end a corresponding current is impressed on the motor. This current generates a magnetic field, which counteracts the field generated by the permanent magnets.
However, the field weakening current can no longer be maintained in the event of a malfunction. Depending on the type of malfunction, impermissible torques, specifically braking torques, and impermissible stresses can then be produced and these then have to be controlled.
In industrial drives a braking torque is not normally a problem in the event of a malfunction, as the best possible fast deceleration is always desirable.
To decelerate an electric motor what is known as armature short circuit braking is known per se. Such deceleration is preferably considered when the motor can no longer be braked electrically (in any other manner), as the short circuit brought about produces a temporarily high current flow. With armature short circuit braking—or in short with an armature short circuit—in the event of a malfunction, the three upper or three lower transistors (IGBTs) of the inverter are activated. The stator windings of the motor are short circuited in the process so that current can flow between them. The rotor is thus brought to a standstill. A braking torque hereby results over the entire speed range, which is still acceptable with typical motor designs.
However the probability that an IGBT module or an activation circuit provided for its activation will fail in such a manner that an armature short circuit is no longer possible is relatively high or at least so high that it is not possible to meet safety requirements in this manner.
In the event of a defect in an IGBT module it is essential to start with all conceivable malfunctions in respect of the transistors and freewheeling diodes contained in the modules.
If for example neither the lower nor the upper transistors can be activated, the problem arises that normally the electromotive force (EMF) of the motor would increase by certain factors in relation to the nominal voltage at high speeds after elimination of the demagnetization current and with the motor connectors open. This is prevented by clamping the stator voltage across the freewheeling diodes to the battery voltage but this produces the exceptional situation of a high recovery current into the battery, generating a high, impermissible braking torque. Once the speed has dropped so low that the rectified EMF is lower than the battery voltage, the braking torque becomes zero.
If on the one hand the transistors can no longer be activated for an armature short circuit and on the other hand the freewheeling diodes are no longer conducting, a very high voltage occurs, also as a further exceptional situation, at the motor connectors or at the output of the inverter. This can cause the destruction of the inverter and/or of the motor as well as arcing with fire as a result and can eliminate the safe electrical isolation of inverter and motor.
It would therefore be desirable and advantageous to obviate prior art shortcomings and to provide an improved method and/or an apparatus for operating a drive system, with which the abovementioned errors can be controlled with adequate reliability, and more particularly a method and/or an apparatus suitable for use in or with electric vehicles.